The pivotal role of Enzymology in Inherited Metabolic Diseases
There are currently more than 1,500 known inherited metabolic diseases, and as a group, they represent the leading cause of child mortality in the Netherlands. One in four children with a metabolic disorder does not reach the age of 18. More than 10,000 families are affected by the profound consequences of these diseases. In these conditions, the chemical factory of the cell and therefore the production and breakdown of building blocks and energy supply, is disrupted.
In metabolic diseases, a particular enzyme does not function properly, which can have serious consequences for metabolism. Enzymology allows us to determine which enzyme is malfunctioning and where precisely metabolism is failing. Being able to predict whether a patient will develop a mild or severe phenotype is important for the treating physician, and enzymology can help assist whether or not the patient should be treated and, if so, how.
Mucopolysaccharidose type 1
This is particularly the case for patients with mucopolysaccharidosis type 1 (MPS1). MPS1 is a lysosomal storage disorder which is included in the newborn screening program in the Netherlands, where newborn babies are tested for a number of inherited conditions that require treatment or intervention in the earliest stages of life to prevent serious complications or premature death.
MPS1, also known as Hurler-Scheie syndrome, is caused by a defect in the enzyme α-iduronidase. The severity of MPS1 can vary, depending on the degree of enzyme deficiency, from mild - called Scheie syndrome - to severe, known as Hurler syndrome. Patients with Scheie syndrome usually exhibit mild symptoms, such as joint stiffness, slight facial abnormalities, and reduced mobility. In patients with Hurler syndrome, brain function deteriorates at a certain point due to the accumulation of waste in the brain. Without treatment, these Hurler patients have a limited life expectancy and usually die in childhood.
Establish disease severity
For a newborn with a MPS1 deficiency, it is important for the physician to know whether the patient has the mild or severe form of MPS1, as this can have significant consequences for the patient and treatment. In the majority of cases, DNA analysis of the α-iduronidase gene, in conjunction with observed clinical abnormalities, can establish the severity of the disease, but not always. In those cases, sensitive enzyme diagnostics could help to determine whether the patient can be treated with enzyme replacement therapy, where a recombinant α-iduronidase is administered to the patient, or whether the more intensive and higher-risk stem cell transplantation is necessary.
Enzymology and new metabolic diseases
Enzymology is not only important for diagnosing known metabolic defects but can also help identify new metabolic diseases. When a metabolic disorder is suspected, genetic diagnostics, especially exome sequencing, are often used first. This technique enables the identification of alterations in more than 22,000 genes that could be responsible for the disease, allowing for an accurate diagnosis. This approach is successful in about 50 percent of patients with a suspected metabolic disorder, but not in the other half.
Glutaminase deficiency?
This was also the case with three young children, all of whom had delayed speech and language development and problems with balance and coordination. The only abnormality found in these three patients was a significantly elevated amount of the amino acid glutamine in their blood, which could suggest a deficiency in the enzyme glutaminase. This enzyme converts glutamine into glutamate, and if it is not functioning properly, glutamine can accumulate. However, a glutaminase deficiency had not been previously identified, and exome sequencing provided no indications for a glutaminase deficiency.
Genomic analysis
Therefore, we developed a method in our laboratory to measure glutaminase activity, which showed that this enzyme indeed does not function well in these three children. Subsequently, we studied the chemical processes in the patients' cells and analyzed the entire genome. In this way, we discovered that a piece of DNA, just before the so-called glutaminase gene, had multiplied, causing the gene to be switched off. With the multiplication, the DNA fragment had extended eight hundred to twelve hundred times its normal length. Such an extension is a unique finding, as diseases usually caused by deletions or loss of function variants in DNA.
To detect such extensions, you need to search among billions of DNA fragments—the proverbial needle in the haystack. Thanks to enzymology we knew where to look for the needle in this haystack, and it is the first time this type of DNA abnormality has been identified as the cause of a metabolic disease. In 2019, we published our study in the prestigious scientific journal The New England Journal of Medicine.
Conclusion
With the increasing use of DNA screening strategies, where large portions of the genome are analyzed, the likelihood of finding new or unclear variants also increases. The availability of enzymology is crucial as it provides direction for genome research: it allows for definitive conclusions whether or not a DNA change results in reduced enzyme function. Thus, the availability of enzymology enables us to provide personalized and improved care for each individual patient.